From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E30FEFD8761 for ; Tue, 17 Mar 2026 12:43:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=JiFrO+qdzRBS30UpjlcTyT9sGWUvl9N5cpLb/NIGpgU=; b=i7fEJq/we9SB5PG2Cnjl8zFtSm n/WN4ovDwi5YYpxaiadXnXn+NYEhgb7nqm+4Urv34vcmpMMOoSIAcfwDN5z/XGeKfE4YdIBcMDyoP tqbZ88lZs4C2bWmLvTTlOP0LDbJg5s7vFc6CZ6duoPs7MFNsqY95uEke5yWdPxHKZw8TpzhVNWXw9 mIX6tlDLskOlKHh62Ty7ODwdv/WxzzuX+xJiFJx/yMeZuEoV3L7gm0xnsO2ryzYjvc3hFV8YlAdOt WiFZLNc/EyQlg1mIutHVf01Z+zrdSN2/2fixY/F5cz1y5UBu8isg76U/qjv0owpCFcgch6cZQMDJO Zv8WUa6w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1w2TlU-00000006K1w-3xkI; Tue, 17 Mar 2026 12:43:16 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1w2TlR-00000006K11-3pEO for linux-arm-kernel@lists.infradead.org; Tue, 17 Mar 2026 12:43:15 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5B2B716F2; Tue, 17 Mar 2026 05:43:05 -0700 (PDT) Received: from [10.57.60.143] (unknown [10.57.60.143]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5C9473F7BD; Tue, 17 Mar 2026 05:43:08 -0700 (PDT) Message-ID: <3d2e8e41-8b41-4d1d-9292-de90425708ec@arm.com> Date: Tue, 17 Mar 2026 13:43:05 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 0/5] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full To: Ryan Roberts , Yang Shi , Jinjiang Tu , catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, david@redhat.com, lorenzo.stoakes@oracle.com, ardb@kernel.org, dev.jain@arm.com, scott@os.amperecomputing.com, cl@gentwo.org Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20250917190323.3828347-1-yang@os.amperecomputing.com> <0b2a4ae5-fc51-4d77-b177-b2e9db74f11d@huawei.com> <0a740020-4780-4156-a9c5-f8b4ada9c8c0@os.amperecomputing.com> <4ad2ea40-b23b-4231-a0de-585b205865c5@arm.com> <9dded616-989b-4846-8596-1c45a6304d36@arm.com> <6c0ed052-5f3c-405a-b53f-4ea21a24479d@arm.com> From: Kevin Brodsky Content-Language: en-GB In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260317_054314_074335_C3CACFCC X-CRM114-Status: GOOD ( 15.05 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 17/03/2026 12:45, Ryan Roberts wrote: > On 17/03/2026 09:29, Kevin Brodsky wrote: >> On 17/03/2026 10:13, Ryan Roberts wrote: >>>>>> Another option would be to initially map by pte then collapse to >>>>>> block mappings >>>>>> once we have determined that all cpus support BBML2_NOABORT. We >>>>>> originally opted >>>>>> not to do that because it's a tax on symetric systems. But we could >>>>>> throw in the >>>>>> towel if it's the least bad solution we can come up with for solving >>>>>> this. I >>>>>> think it might help some of Kevin's use cases too? >>>>> May be an option too. When we discussed this there was no usecase for >>>>> direct mapping collapse. But if we can have multiple usecases, it may >>>>> be worth it. >>> I could imagine that if user space creates and destroys lots of secretmem areas, >>> then it will completely split the linear map to ptes and that will never recover >>> currently. So I think in the long term, having the ability to collapse would be >>> useful. I just don't particularly like forcing symetric systems to map by pte >>> initially (which is slow) only to collapse later (which will cost even more >>> time). But it does feel inherrently more robust. >> Now that you spell it out, I'm realising this would actually make things >> pretty complicated for protected page tables. In that series, page >> tables for the linear map are allocated by a separate memblock-based >> allocator [1], tracking the allocated ranges to set their pkey later. >> There's a strong assumption that these page tables are never freed. >> >> If we initially PTE-mapped the linear map and then later collapsed it, >> that assumption clearly wouldn't hold. > Sorry I don't understand why the assumptions change? All I'm proposing is walkng > the linear map to find compatible PTEs and collapsing them into the biggest > possible blocks. The pages aren't being freed, they are just being mapped > differently (which can be done live for BBML2_NOABORT). PTEs with different > pkeys would be considered incompatible, so we would end up with a boundary in > the leaf mappings at that point. I'm not sure I'm following, if all entries in a PTE page are compatible, then surely we just convert the parent PMD entry to become a leaf and then free the PTE page? And same idea one level above. > >> It could be handled by poking >> holes in the tracked ranges, but it gets ugly and increases fragmentation. > You'd still want page tables to be allocated from contiguous physical (and > virtual) memory so that the boundaries where pkeys change are minimized. Yes that's for sure, that's why I'm concerned with individual pages being freed in a middle of a block. > I guess I've misunderstood something... I might have too :/ - Kevin